Examples of the fields of information recording, to which an apparatus for producing a thin film and a process for producing a thin film are applied, include a protective layer formed on a magnetic recording medium, such as a magnetic tape and a magnetic disk, used in particular as an information recording medium. Such a protective layer is demanded to have an increased throughput of the production process thereof and an increased wear resistance of the protective layer produced by the process. A conventional example of an apparatus for producing a thin film and a process for producing a thin film, which are applied to a magnetic recording medium, such as a magnetic tape, will be described below.
As a magnetic recording medium, a coating type magnetic recording medium has been widely employed, which is produced by coating a magnetic coating composition comprising a powder magnetic material, such as oxide magnetic powder and alloy magnetic powder, dispersed in an organic binder, such as a vinyl chloride-vinyl acetate series copolymer, a polyester resin, a urethane resin and a polyurethane resin, on a non-magnetic support, followed by drying.
Along with increase in demand for high density magnetic recording, on the other hand, a magnetic recording medium having a so-called metallic thin film type magnetic recording layer is being received attention, which is produced by directly adhering a metallic magnetic material, such as a Co-Ni alloy, a Co-Cr alloy and Co-O, on a non-magnetic support of a film form, such as polyethylene terephthalate (PET), polyester, polyamide and polyimide, by plating or vacuum thin film forming means (such as a vacuum deposition method, a sputtering method and an ion plating method).
In recent years, a magnetic tape, called a vapor deposition tape, comprising a metallic thin film type magnetic recording layer widely spreads to a field of a digital video cassette recorder and a field of a data streamer, such as AIT (Advanced Intelligence Tape, a trade name of Sony Corp.), and improvement in film characteristics of a protective layer of the magnetic tape is being earnestly demanded.
An example of a magnetic recording medium having a metallic thin film type magnetic recording layer will be described with reference to FIG. 1 showing a schematic cross sectional view of a magnetic recording medium.
In a magnetic recording medium 4, such as a magnetic tape, a metallic thin film type magnetic recording layer 7 comprising a ferromagnetic metal, such as Fe, Co and Ni, or a ferromagnetic alloy containing one of Fe, Co and Ni is formed on a non-magnetic support 5, such as a polyester film, by a thin film forming method, such as a vacuum deposition method, a sputtering method and an ion plating method. The magnetic recording layer and the non-magnetic support may be a single layer structure or a multi-layer structure. Furthermore, depending on necessity, an undercoating layer 6, a protective layer 8, such as those of hard carbon series, a lubricant layer 9 and a backcoating layer 10 are formed.
In the magnetic recording medium 4 having the metallic thin film type magnetic recording layer 7, as different from a coating type magnetic recording medium, because a binder as a non-magnetic material is not mixed in the magnetic recording layer 7 to increase the packing density of the magnetic material, it exhibits various advantages in that the coercive force and the squareness ratio are excellent, the recording demagnetization and the thickness loss on reproduction are extremely small, and the electromagnetic conversion characteristics at a short wavelength are excellent. That is, it is considered that the magnetic recording medium 4 having the metallic thin film type magnetic recording layer 7 becomes a main stream of high density magnetic recording owing to its excellent magnetic characteristics.
Furthermore, in order to improve the electro-magnetic conversion characteristics of the magnetic recording medium 4 of this type to obtain larger output, a so-called oblique vapor deposition method has been practiced, in which the magnetic recording layer 7 of the magnetic recording medium 4 is vapor-deposited in an oblique manner.
There is a tendency in that the surface of the magnetic recording medium 4 is being smoothed to decrease the spacing loss owing to the stream of increasing recording density. However, it brings about increase in frictional force between a magnetic head not shown in the figure and the magnetic recording medium 4, and increase in shearing stress formed in the magnetic recording medium 4.
In view of the background situation described above, a protective layer 8 is formed on the surface of the magnetic recording layer 7 of the magnetic recording medium 4 to decrease the friction coefficient between a magnetic head and the magnetic recording medium 4 and to increase the wear resistance characteristics.
As the protective layer 8, a carbon film, a quartz (SiO.sub.2) film and a zirconia (ZrO.sub.2) film have been studied, and some of them have been practically produced for a magnetic disk. In particular, a diamond-like carbon film (hereinafter referred to as a DLC film) having a higher hardness is formed as the protective layer 8 in recent years, which is expected to be a main stream. Application thereof to a magnetic tape, such as a vapor deposition tape, which is demanded to have sliding characteristics and environment resistance characteristics, is being studied.
As an apparatus for producing a thin film for producing the protective layer 8, a sputtering apparatus and a plasma CVD (chemical vapor deposition) apparatus are frequently employed. The apparatus for producing a thin film and the process for producing a thin film will be described below referring to a plasma CVD apparatus having an increased film formation rate thereof to improve the throughput.
In the plasma CVD apparatus, a step of forming a thin film is conducted, in which a chemical reaction of a gas as a raw material, such as decomposition and recombination, is conducted by utilizing the energy of plasma generated by an electric field or a magnetic field. While DC discharge and RF discharge are generally used as the plasma source in the plasma CVD apparatus, a method utilizing ECR (electron cyclotron resonance) is being studied.
As an example of the plasma CVD apparatus, the case where the protective layer 8 of the magnetic recording medium 4 in the form of a magnetic tape is produced by the plasma CVD apparatus of a DC discharge type will be described with reference to FIG. 2 showing a schematic cross sectional view of the plasma CVD apparatus.
In the plasma CVD apparatus 11 shown in FIG. 2, a protective film, such as a DLC film, is formed on a material to be treated 12, a magnetic recording medium, such as a magnetic tape. The material to be treated 12 is supplied from a supplying roller 14 while supporting by a pair of guide rollers 1 in a vacuum chamber 13, and supported with winding by a can roller 15 capable of rotating at a constant rate, which functions as a counter electrode and is equipped with a cooling mechanism. The material to be treated 12 is rotated and driven by the can roller 15, and run and stored by winding by a winding roller 16. The vacuum chamber 13 is evacuated to a prescribed atmosphere of a reduced pressure by a vacuum evacuation device 17.
A hydrocarbon series raw material gas introduced into a reaction tube 18 through gas inlet 19 is converted to a plasma with a DC electric field of from 500 to 2,000 V through an electrode 20 provided inside the reaction tube 18, and subjected to reactions, such as decomposition and recombination, to form a film, such as a carbon film, on the material to be treated 12 carried by the can roller 15. An electric power is supplied to the electrode 20 from a DC power source 21. The durability of a magnetic recording medium of the material to be treated 12 is considerably improved by forming the protective layer, such as a carbon film, by using the plasma CVD apparatus 11.
However, as shown in FIG. 3 showing a schematic cross sectional view of an important part of the plasma CVD apparatus, in the case where a film forming material having been plasma-decomposed by application of a high voltage is formed as a film on the material to be treated 12, a large electric current flows on the material to be treated 12 owing to the constitution of the apparatus.
The large electric current thus formed brings about, unless it is grounded, problems in that normal running of the material to be treated 12 becomes impossible, and in the case where the material to be treated 12 is in contact and sliding with the guide roller 1 made of a metal, arc discharge as small lightening discharge occurs to damage the material to be treated 12 itself (in the case where the material to be treated 12 is a magnetic recording medium, micro-deformation of the magnetic layer and the non-magnetic support called pinhole type heat degradation frequently occurs). There is a further problem in that the problems become conspicuous when a higher voltage is applied (which is proportional to the film forming rate).